全文获取类型
收费全文 | 12743篇 |
免费 | 1092篇 |
国内免费 | 6篇 |
出版年
2023年 | 72篇 |
2022年 | 143篇 |
2021年 | 275篇 |
2020年 | 147篇 |
2019年 | 178篇 |
2018年 | 266篇 |
2017年 | 233篇 |
2016年 | 401篇 |
2015年 | 625篇 |
2014年 | 797篇 |
2013年 | 864篇 |
2012年 | 1197篇 |
2011年 | 1080篇 |
2010年 | 706篇 |
2009年 | 652篇 |
2008年 | 827篇 |
2007年 | 801篇 |
2006年 | 755篇 |
2005年 | 754篇 |
2004年 | 667篇 |
2003年 | 597篇 |
2002年 | 571篇 |
2001年 | 119篇 |
2000年 | 77篇 |
1999年 | 113篇 |
1998年 | 124篇 |
1997年 | 86篇 |
1996年 | 89篇 |
1995年 | 67篇 |
1994年 | 56篇 |
1993年 | 66篇 |
1992年 | 49篇 |
1991年 | 39篇 |
1990年 | 32篇 |
1989年 | 32篇 |
1988年 | 24篇 |
1987年 | 14篇 |
1986年 | 17篇 |
1985年 | 13篇 |
1984年 | 21篇 |
1983年 | 22篇 |
1982年 | 11篇 |
1981年 | 14篇 |
1980年 | 17篇 |
1979年 | 13篇 |
1978年 | 10篇 |
1977年 | 11篇 |
1976年 | 12篇 |
1973年 | 7篇 |
1972年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Helicobacter pylori is an important etiologic agent of gastroduodenal disease in humans. In this report, we describe a general genetic approach for the identification of genes encoding exported proteins in H. pylori. The novel TnMax9 mini-blaM transposon was used for insertion mutagenesis of a H. pylori gene library established in Escherichia coli. A total of 192 E. coli clones expressing active β-lactamase fusion proteins (BlaM+) were obtained, indicating that the corresponding target plasmids carry H. pylori genes encoding putative extracytoplasmic proteins. Natural transformation of H. pylori P1 or P12 using the 192 mutant plasmids resulted in 135 distinct H. pylori mutant strains (70%). Screening of the H. pylori collection of mutant strains allowed the identification of mutant strains impaired in motility, in natural transformation competence and in adherence to gastric epithelial cell lines. Motility mutants could be grouped into distinct classes: (i) mutant strains lacking the major flagellin subunit FlaA and intact flagella (class I); (ii) mutant strains with apparently normal flagella, but reduced motility (class II), and (iii) mutant strains with obviously normal flagella, but completely abolished motility (class III). Two independent mutations that exhibited defects in natural competence for genetic transformation mapped to different genetic loci. In addition, two independent mutant strains were isolated by their failure to bind to the human gastric carcinoma cell line Katoill. Both mutant strains carried a transposon in the same gene, 0.8 kb apart, and showed decreased autoagglutination when compared to the wild-type strain. 相似文献
92.
93.
Torsten Müller Stefan Fiedler Thomas Schnelle Kai Ludwig Hartmut Jung Günter Fuhr 《Biotechnology Techniques》1996,10(4):221-226
Summary Combining dielectrophoretic and hydrodynamic forces in micro electrode structures allows enrichment and stable trapping of viruses in aqueous solutions. Fluorescently labelled Influenza and Sendai viruses were collected from solutions of 2*105 – 2*108 viruses/l within a few seconds. In the central part of the trap a virus aggregate of about 2–9 m in diameter was formed. This corresponds to a local enrichment of viruses up to a factor of about 1400. 相似文献
94.
Peptide motifs of HLA-B38 and B39 molecules 总被引:2,自引:2,他引:0
95.
The microreticulate resting cyst of the potentially toxic, chain-forming,unarmoured neritic dinoflagellate Gymnodinium catenalum Graham1943. the planktonic stage of which is not known from NorthEuropean waters, is reported for the first time from recentGerman coastal sediments of the North Sea and Baltic Sea. Insandy mud sediments of the German Bight, a maximum of 8 5 livingcysts cm3 were found. In Kiel Bight sediments G.catenalumwas found in maximum concentrations of 17.0 living cysts cm3.In surface waters of the German Bight resuspended G catenatumcysts were observed at concentrations of up to 3.6 cysts l1.Successful germination experiments conducted with natural seawatershow that the occurrence of a vegetative form of G.catenatumin northern Europe is very likely. The present study highlightsthat cyst surveys provide an important tool for the evaluationof areas with potential toxicity problems, as they may indicatethe presence of hitherto overlooked species in the water column. 相似文献
96.
Stefan Falk Guy Samson Doug Bruce Norman P. A. Huner David E. Laudenbach 《Photosynthesis research》1995,45(1):51-60
Under conditions of iron-stress, the Photosystem II associated chlorophyll a protein complex designated CP 43, which is encoded by the isiA gene, becomes the major pigment-protein complex in Synechococcus sp. PCC 7942. The isiB gene, which is located immediately downstream of isiA, encodes the protein flavodoxin, which can functionally replace ferredoxin under conditions of iron stress. We have constructed two cyanobacterial insertion mutants which are lacking (i) the CP 43 apoprotein (designated isiA
–) and (ii) flavodoxin (designated isiB
–). The function of CP 43 was studied by comparing the cell characteristics, PS II functional absorption cross-sections and Chl a fluorescence parameters from the wild-type, isiA
– and isiB
– strains grown under iron-stressed conditions. In all strains grown under iron deprivation, the cell number doubling time was maintained despite marked changes in pigment composition and other cell characteristics. This indicates that iron-starved cells remained viable and that their altered phenotype suggests an adequate acclimation to low iron even in absence of CP 43 and/or flavodoxin. Under both iron conditions, no differences were detected between the three strains in the functional absorption crossection of PS II determined from single turnover flash saturation curves of Chl a fluorescence. This demonstrates that CP 43 is not part of the functional light-harvesting antenna for PS II. In the wild-type and the isiB
– strain grown under iron-deficient conditions, CP 43 was present in the thylakoid membrane as an uncoupled Chl-protein complex. This was indicated by (1) an increase of the yield of prompt Chl a fluorescence (Fo) and (2) the persistence after PS II trap closure of a fast fluorescence decay component showing a maximum at 685 nm.Abbreviations Chl
chlorophyll
- CP 43, CP 47 and CP 43
Chl a binding protein complexes of indicated molecular mass
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- Fm and Fm
fluorescence when all PS II reaction centers are dosed in dark- and light-acclimated cells, respectively
- Fo
fluorescence when all PS II reaction centers are open in dark acclimated cells
- Fv
variable fluorescence after dark acclimation (Fm–Fo) 相似文献
97.
Anton Stefan Reiter und Gerhard Loupal 《Journal of Ornithology》1995,136(2):221-223
In July 1992, in the Austrian part of Hanság, a seventy day old young bustard was found dead in a grassland. On the left intertarsal joint a walnut sized open pock was located. Other pocks reaching pea-size were found on both legs. The diagnosis pox was established by light- and electron-microscopic examination of the lesions. A further chick of another hen, fledged in the same year, observed from a distance showed abnormal thickening of the intertarsal joint area. The consequences of pox for such a small group of Great Bustards (total for 1988–1993 15–20 birds) should be watched carefully. 相似文献
98.
Feeding biology of two brachionid rotifers: Brachionus quadridentatus and Brachionus plicatilis 总被引:1,自引:0,他引:1
Brachionus quadridentatus and Brachionus plicatilis were examined in laboratory cultures to investigate their (1) food requirements and (2) ability to select particles on the basis of size. Growth experiments showed that the Chlorophyceans Nannochloris sp. and Chlorella vulgaris possess good nutritional value for these rotifers. However, B. quadridentatus declined in cultures with Microcystis firma, detritus of Enteromorpha sp. or Pseudomonas sp. (bacteria) as a sole source of food, thus indicating that these foods were inadequate. Utilization of latex microspheres by these two species indicate that they respond to particle size in different ways: B. quadridentatus had a higher selectivity index for 3 and 5 µm beads, while B. plicatilis had a higher selectivity index for 2 µm beads. 相似文献
99.
Uwe-G. Maier Stefan A. Rensing Gabor L. Igloi Martina Maerz 《Molecular & general genetics : MGG》1995,246(1):128-131
Introns within introns (twintrons) are known only from the Euglena chloroplast genome. Twintrons are group II or III introns, into which another group II or III intron has been transposed. In this paper we describe a non-Euglena twintron structure within a plastid-encoded chaperone gene (cpn60) of the cryptomonad alga Pyrenomonas salina. In addition, the evolutionary relationships between members of the Cpn60 protein family are determined. Our findings permit the inclusion of cryptomonad plastomes in phylogenetic studies of intron evolution and present further evidence for the origin of modern plastids from a cyanobacterial ancestor.This paper is dedicated to Prof. Dr. Peter Sitte on the occasion of his 65th birthday 相似文献
100.
Nicolae Barbacar Stefan Hinnisdaels Isabelle Farbos Françoise Monéger ré Lardon Catherine Delichère Armand Mouras Ioan Negrutiu 《The Plant journal : for cell and molecular biology》1997,12(4):805-817
The dioecious white campion (Silene latifolia) has been chosen as a working model for sexual development. In this species, sexual dimorphism is achieved through two distinct developmental blocks: inhibition of carpel development in male flowers, and early arrest of anther differentiation in female flowers. The combined advantages of the dioecious system and the availability of a sexual mutant lacking both male and female reproductive organs have been exploited in a molecular subtraction approach using male and asexual flower buds. This resulted in the cloning of 22 cDNA clones expressed in stamens at distinct stages of development. Fourteen of these clones corresponded to genes whose expression was detected in pre-meiotic stamens, a stage of development for which very little information is presently available. Furthermore, the absence of similarities with database sequences for ten clones suggests that they represent novel genes. Functional analysis of each clone will enable their positioning within the reproductive organ developmental pathway(s). In parallel, these clones are being exploited as developmental markers of early differentiation within the flower. 相似文献